In microwave and millimeter-wave systems, transmission of quasi-optical electromagnetic radiation is typically accomplished by use of conventional guided-wave conductors, such as a waveguide or microstrip, or by free-space transmission. These systems frequently employ electronically controllable devices that provide the capability of either selectively reflecting or transmitting quasi-optical radiation, but not both. While mechanical switching devices capable of switching between transmission and reflection modes are available, an electronically controllable switch is highly desirable for various imaging and processing applications.
An example of a system in which such a switching capability would be desirable is the millimeter-wave imaging system disclosed in U.S. patent application Ser. No. 07/495,879, now U.S. Pat. No. 5,047,783. This system employs a receiver array for receiving millimeter wave signals and for generating images of a selected field of view. To reduce noise and thereby improve image quality, a noise calibration signal from a uniform load is subtracted from the signal emanating from the desired field of view. For this reason, the system alternates the beam incident on the receiver array between the beam radiating from the field of view and the beam radiating from the uniform load. This alternation between energy sources is accomplished by the coaction between a polarizing grid and a mechanically rotatable polarization element located in the path of the beams from the field of view and the uniform load. The grid reflects radiation polarized in one direction and transmits radiation polarized in a direction perpendicular thereto. The grid is positioned so that it transmits the uniform load radiation and reflects the field of view radiation in the direction of the polarization element and the receiver array. Accordingly, energy radiating from the uniform load and reflected energy radiating from the field of view are polarized in orthogonal directions with respect to one another. The polarization element is aligned to pass radiation oriented in one of these directions. However by rotating the polarization element by 90.degree., the polarization element passes the radiation polarized in the second direction. As a result, if the polarization element is continuously rotated, the array alternately detects radiation from the field of view and radiation from the uniform load.
An electronically controllable device that selectively transmits incident radiation from one source or another would significantly simplify this apparatus, permitting the elimination of the rotating polarization grid as well as the mechanism for rotating it.
Presently, electronically controllable devices have been developed only for phase-shifting or deflecting quasi-optical electromagnetic energy. In some of these devices, the phase-shifting or deflecting capabilities are provided by a grid of wires embedded in a dielectric, wherein the wires are divided into sections by switches, such as diodes. An overview of such prior devices is provided below.
U.S. Pat. No. 3,708,796 issued to Gilbert relates to a dielectric panel for phase-shifting an electromagnetic beam travelling via free space by interposing one or several of the dielectric panels in the path of the beam. Each panel comprises conductive leads running parallel with the electric field vector of the incident wave. The leads are connected by silicon diodes and can either be electrically divided into sections or electrically connected along the length of the panel depending on the bias state of the diodes. The distance between the sections of the leads is such that a wave incident on the panel does not induce any current in the leads. According to Gilbert, this results in a substantial phase shift of the incident wave as it traverses the panel. Further, the disclosed panel has a thickness which is a multiple of a half wavelength of the incident wave, so as to prevent any back-reflection of the incident wave. Thus, the '796 patent does not disclose a means of reflecting an electromagnetic beam. On the contrary, the panels are designed so as to prevent reflections. As will become apparent, the absence of any reflection capability is one distinguishing feature from the present invention.
Chekroun et al., "Radant: New Method of Electronic Scanning", Microwave Journal, pp. 45-53 (Feb. 1981) and Park, "Radant Lens: Alternative to pensive Phased Arrays", Microwave Journal, pp. 101-105 (Sept. 1981) both describe a lens comprising a grid of wires, which is capable of electronically varying the phase of an electromagnetic beam incident on the lens. The wires are separated into sections by PIN diodes. The phase of the beam transmitted through the lens is varied by varying the DC bias of the diodes. The lens is functionally equivalent to the phase-shifting device disclosed in the '796 patent described previously. However, as with the '796 patent, these articles do not disclose a device for the selective reflection or transmission of an electromagnetic beam because the disclosed devices do not provide for the reflection of electromagnetic energy.
Lam et al., "Diode Grids for Electronic Beam Steering and Frequency Multiplication", International Journal of Infrared and Millimeter Waver, Vol. 7, No. 1, pp. 27-41 (1986) relates to two arrays comprising diode grids. The first array controls the phase shift of a wave reflected by the array so as to steer the reflected beam in a desired direction. This array comprises a fused-quartz cover, two diode grids and a metal mirror. A wave incident on the array is reflected and re-directed by controlling the phase shift of the reflected wave. The phase shift of the reflected wave is determined by the capacitance characteristic of the diodes in the grids, and this capacitance characteristic is controlled by varying the DC bias of the diodes. In the first array disclosed by Lam et al., there is a significant reflection loss of the incident radiation. The reflection efficiency of the beam steering array varies from 0.49 to 0.57, with an average loss of 2.7 dB. Furthermore, this array does not transmit electromagnetic energy through the array because any incident energy is always reflected by the metal mirror.
The second array described by Lam et al. generates the second harmonic of the fundamental frequency of an electromagnetic beam incident on the array. The second array comprises an input filter, a diode grid and an output filter. The input filter allows the transmission of the fundamental frequency and prevents the transmission of the second harmonic of the incident beam. The energy incident on the diode grid in conjunction with the non-linear capacitance of the diodes generates harmonics, whereby the output filter is tuned to allow the transmission of the second harmonic of the incident beam. This array does not permit the reflection of incident radiation. Thus, neither one of the arrays disclosed by Lam et al. can be employed to selectively reflect or transmit incident radiation.
U.S. Pat. No. 4,581,250 and U.S. Pat. No. 4,754,243 issued to Armstrong, et al. relate to a method of mounting a microwave semiconductor device in a radio-frequency (RF) waveguide, whereby a matrix of PIN diodes operates as a high power limiter. In a first state, the diode matrix allows for the transmission of energy through the waveguide and in a second state the diode matrix prevents the transmission of energy through the waveguide. However, the Armstrong patents do not disclose a state of device operation in which substantially all the energy incident on the diode matrix is reflected.
Although the aforementioned references relate to electronically controllable devices for manipulating an electromagnetic beam incident on a device, none of the above references disclose a device capable of reflecting substantially all of the incident electromagnetic radiation in a first state and transmitting the incident radiation in a second state.
Accordingly, there exists a present need for a device which provides for the transmission of electromagnetic radiation in one state and for the reflection of the energy in another state, wherein the device comprises means for electronically switching between the two states.